Sweat sensing technologies have enormous potential for applications ranging from athletics, to neonatology, to pharmacological monitoring, to personal digital health, to name a few applications. Sweat contains many of the same biomarkers, chemicals, or solutes that are carried in blood and can provide significant information enabling one to diagnose illness, health status, exposure to toxins, performance, and other physiological attributes even in advance of any physical sign. Furthermore, sweat itself, the action of sweating, and other parameters, attributes, solutes, or features on, near, or beneath the skin can be measured to further reveal physiological information.
If sweat has such significant potential as a sensing paradigm, then why has it not emerged beyond decades-old usage in infant chloride assays for Cystic Fibrosis or in illicit drug monitoring patches? In decades of sweat sensing literature, the majority of practitioners in the art use the crude, slow, and inconvenient process of sweat stimulation, collection of a sample, transport of the sample to a lab, and then analysis of the sample by a bench-top machine and a trained expert. This process is so labor intensive, complicated, and costly that in most cases, one would just as well implement a blood draw since it is the gold standard for most forms of high performance biomarker sensing. Hence, sweat sensing has not emerged into its fullest opportunity and capability for biosensing, especially for continuous or repeated biosensing or monitoring. Furthermore, attempts at using sweat to sense “holy grails” such as glucose have not yet produced viable commercial products, reducing the publically perceived capability and opportunity space for sweat sensing.
Of all the other physiological fluids used for bio monitoring (e.g., blood, urine, saliva, tears), sweat has arguably the least predictable sampling rate in the absence of technology. However, with proper application of technology, sweat can be made to outperform other non-invasive or less invasive biofluids in predictable sampling.
For example, it is difficult to control saliva or tear rate without negative consequences for the user (e.g., dry eyes, tears, dry mouth, or excessive saliva while talking). Urine is also a difficult fluid for physiological monitoring, because it is inconvenient to take multiple urine samples, it is not always possible to take a urine sample when needed, and control of biomarker dilution in urine imposes further significant inconveniences on the user or test subject.
Known and existing methods of reducing sweat volume and increasing sampling rate predictability include those reported frequently in the clinical literature, such as coating the skin with petroleum jelly or oil through which sweat can push. However, these techniques have been demonstrated only for sweat collection and are not necessarily compatible with a wearable sensor. For example, petroleum jelly would wet against the sensor and effectively seal it from any sweat. Furthermore, other possible sweat pressure-activated methods must somehow be affixed to skin so that sweat is confined horizontally (otherwise sweat pressure activation is not possible). Conventional approaches will not work with wearable sensors, and inventive steps are required for enablement. Clearly, the state of art is lacking in devices to properly reduce the volume between sensors and skin, which is critical for fast sampling times or for sampling during intervals with very low sweat rates. In addition, it also may be critical for prolonged stimulation (i.e., where less stimulation is required over longer periods), and for improving biomarker measurements where a low sweat rate is required to ensure correlation between biomarker concentrations in sweat and those in blood.
One novel method of reducing sweat volume as disclosed in PCT/US2016/043771 involves using pressure-activated sealants to horizontally confine sweat flow and reduce sweat volume. In order to reduce sweat volume, however, sweat pressure-activated methods also require the sensor to be properly aligned with sweat glands, which can prove difficult. Since it would be impractical for sweat sensing device users to reliably place a device in ideal alignment with sweat glands, devices may be designed to optimize sweat gland coverage when the device is randomly placed on skin. However, even with such designs, sweat gland density may vary with between individuals, or even body location on the same individual. Therefore, a sweat sensing device that is self-aligning with sweat glands may improve sensor proximity to sweat glands under a variety of circumstances, thereby reducing sweat volume.
However, self-aligning sweat sensing designs must also be configured to access prolonged sweat stimulation, which is a significant challenge. Further, as with other referenced means of reducing sweat volume, self-aligning sensors must also be protected from abrasion. The disclosed invention, therefore, discloses a means of providing prolonged sweat stimulation for abrasion-protected self-aligning sensors by configuring a sweat-stimulating chemical in close proximity to the sensors, and enabling sudomotor axon reflex sweat response through diffusion of the sweat stimulation compound into the skin.
Many of the drawbacks and limitations stated above can be resolved by creating novel and advanced interplays of chemicals, materials, sensors, electronics, microfluidics, algorithms, computing, software, systems, and other features or designs, in a manner that affordably, effectively, conveniently, intelligently, or reliably brings sweat sensing technology into intimate proximity with sweat as it is generated. With such an invention, sweat sensing could become a compelling new paradigm as a biosensing platform.